When exposed to radiation such as X-rays, an energy-storing phosphor (e.g., stimulable phosphor, which gives off stimulated emission) absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the storage panel; sequentially scanning the storage panel with stimulating light such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The radiation image storage panel thus treated is generally subjected to a step for erasing radiation energy remaining therein. In the erasing step, the panel is irradiated with light (i.e., erasing light) for erasing (or releasing) the remaining radiation energy. After the erasing step is complete, the storage panel is stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and a phosphor layer provided thereon. If the phosphor layer is self-supporting, however, the support may be omitted. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer so as to keep the phosphor layer from chemical deterioration or physical damage.
Various kinds of energy-storing phosphor layers are known. For example, the phosphor layer can comprise a binder and energy-storing phosphor particles dispersed therein, or otherwise can comprise agglomerate of an energy-storing phosphor without binder. The latter layer can be formed, for example, by a gas phase-accumulation method, in which a phosphor is vaporized or sputtered under vacuum so that the phosphor would be deposited and accumulated on a substrate to form a layer of the phosphor in the form of columnar crystals. The phosphor layer thus formed by a gas phase-accumulation method contains no binder and consists of only the phosphor, and there are gaps among the columnar crystals of the phosphor. Because of these gaps, the stimulating light can stimulate the phosphor efficiently and further the emitted light can be collected efficiently. Accordingly, a radiation image storage panel having an energy-storing phosphor layer comprising a phosphor in the form of columnar crystals has high sensitivity. At the same time, since the gaps prevent the stimulating light from diffusing parallel to the phosphor layer, the storage panel can give a reproduced image of high sharpness.
In the radiation energy erasing step of the radiation image recording and reproducing method (or radiation image forming method), the amount or energy of erasing light in the erasing step is generally determined according to reading sensitivity (dynamic range) or intensities of signals obtained in the reading procedure.
JP-A-6-175243 discloses a radiation image-reading apparatus equipped with a means for erasing a radiographic latent image and a means for controlling the erasing. The erasing means comprises an erasing light source for emitting erasing light, and the controlling means controls the amount and/or scanning speed of the erasing light according to the dynamic range of reading. Since the dynamic range is generally determined to a level larger than the maximum signal to be read out, the amount of erasing light determined on the dynamic range often has excessively large energy. Consequently, it is difficult to reduce energy consumption of the erasing light source in the apparatus.
U.S. Pat. No. 5,818,065 discloses a radiation image reading method and an apparatus thereof in which an energy-storing phosphor sheet after subjected to the erasing step is again subjected to the reading procedure. If the signals obtained in the second reading procedure is stronger than a predetermined value, the sheet is again subjected to the erasing step. In the disclosed method and apparatus, it is necessary to perform the reading procedure twice, and further if the remaining energy is insufficiently erased, it is necessary to perform again the erasing step. Accordingly, a reading system adopting the disclosed method has poor throughput, and parts of the disclosed apparatus such as a laser source for stimulation, a signal detector (e.g., CCD) and an erasing light source show short life times.
JP-A-2001-33902 discloses a method for determining an amount of energy required to erase the remaining radiation energy. In the disclosed method, the required energy level is estimated from the maximum light amount (maximum signal intensity) of stimulated emission. If the emission is stronger than the largest intensity level measurable in the reading system, the maximum light amount is assumed on the basis of, for example, a histogram beforehand prepared according to the desired radiation image, and the energy level of erasing light is determined from thus obtained maximum light amount. This method needs a means for determining the energy level of erasing light on the basis of the obtained signals.
JP-A-2001-74898 discloses a radiation image reading method and an apparatus thereof in which a stimulable phosphor screen comprising a divalent europium activated cesium halide (chloride or bromide) phosphor and having a surface area not larger than Smax is exposed to erasing light emitted from an erasing light source having an electric power not larger than Smax×1 J, so as to erase radiation energy remaining in the screen. In this method, the irradiation power of erasing light is determined independently of the exposure of radiation but from the surface area of the screen, and hence too much erasing energy would be required. Further, the degree of erasing depends on not only the irradiation power but also the amount (i.e., illuminance and irradiation time) of erasing light received by the screen.
Japanese Patent No. 3,582,041 discloses a radiation image-reading apparatus equipped with an erasing light-controlling means. In the apparatus, if a radiation image storage panel is to be subjected to the reading procedure of higher sensitivity than the prior reading procedure, the erasing light-controlling means makes the storage panel irradiated with erasing light before radiation image information is recorded therein. Further, after the recorded radiation image information is read out with the higher sensitivity in the reading procedure, the storage panel is irradiated again with the erasing light in an amount determined according to that reading sensitivity.
The reading apparatus, by which radiation image information is read out from a radiation image storage panel, is generally equipped with an erasing means for the erasing step. However, it is desired to shorten the time for erasing and thereby to increase throughput of the apparatus. Further, it is also desired to simplify the apparatus and thereby to reduce the cost.